1. Field of the Invention
The present invention relates to a holographic memory device.
2. Description of the Related Art
In recent years, as the demand for recording and later reconstructing the large-sized information is increasing, optical information recording/reconstructing technique using holography, a method of three-dimensional optical image formation, has been developed other than two-dimensional recording/reconstructing devices using optical disk recording media such as CDs, DVDs, etc.
Referring to FIG. 1 showing an information recording/reconstructing apparatus using holography, a laser 1 is used as a light source, and a beam splitter 14 is used to divide a beam 16 from laser 15 into a reference beam 12 and an object beam 7.
The reference beam 12 enters a beam deflector 11 which changes the direction of reference beam 12 by reflection or refraction, in order to allow reference beam 12 to be a reflected beam 13.
The reflected beam 13 goes to a telescope 9 of 1:1 in magnification.
An axis of telescope 9 coincides with a predetermined optical axis 21, and the reflected beam 13 is incident on the telescope 9 at a specific angle A1 20 with respect to optical axis 21.
After passing through the telescope 9, the reflected beam 13 is changed into an incident reference beam 6, and then goes to a recording medium 3. Here, the direction of incident reference beam 6 is opposed to that A1 20 of reflected beam 13 with respect to optical axis 21, and the absolute value of incident angle A2 19 of reference beam 6 is the same as the angle A1 20.
When an axis 18 of recording medium 3 coincides with the optical axis 21, the incident reference beam 6 enters the recording medium 3 at an angle of incidence A2 19.
The beam deflector 11 is controlled to adjust the angle A1 20.
A computer 8 controls the beam deflector 11 to adjust the angle A1 20 of reflected beam 13. Therefore, the angle of incidence A2 19 of incident reference beam 6 incident on the recording medium 3 is also controlled by computer 8.
After passing through a shutter 17 for controlling the traveling of beam 16, a reflecting mirror 10 for changing the traveling direction of beam 7, a spatial light modulator (SLM) 1 for presenting the information, and a Fourier transform lens (FTL) 2 for generating an image by Fourier transforming the image displayed on SLM 1, the object beam 7 enters the recording medium 3, in the form of incident object beam 7'. The incident reference beam 6 and the incident object beam 7' meet each other in the recording medium 3.
A collecting lens 4 is on a position spaced a given distance away from recording medium 3 in the traveling direction of object beam 7. A charge coupled device (CCD) 5 is positioned on collecting lens 4's collecting side to change the image of SLM 1 into an electrical signal, and SLM 1 and CCD 5 are controlled by computer 8.
The following description will now relate to the 20 information recording operation of holographic memory.
When the information to be recorded in SLM 1 is displayed by computer 8 and the object beam 7 is incident on recording medium 3, the information displayed on SLM 1 is transformed by FTL 2 so that object beam 7' incident on recording medium 3 holds the Fourier transformed information of SLM 1.
When incident reference beam 6 and incident object beam 7' go to recording medium 3, the pattern of interference between these beams is recorded on recording medium 3.
Another information is displayed on SLM 1, to be recorded on recording medium 3, and the beam deflector 11 is adjusted to change the angle of incidence A2 19 of incident reference beam 6 into A2'.
After passing through SLM 1, incident object beam 27 goes to the recording medium 3 at the same angle as the incident object beam 7', but holds the information different from the previous information due to the different information of SLM 1. The pattern of interference between incident object beam 27 and incident reference beam 6 slightly different from each other in angle of incidence is recorded on recording medium 3.
Several pieces of information can be recorded on recording medium 3 by changing the angle of incidence 19 of incident reference beam 6 in this way.
In operation of reconstructing the information recorded in the above-mentioned way, as object beam 7 is cut off by shutter 17, and reference beam 6 is incident on recording medium 3 at the same angle of incidence as the specific angle of incidence A2 19, reference beam 6 is diffracted by the interference pattern recorded on recording medium 3, and reconstructed object beam 7" traveling toward CCD 5 is created. When this reconstructed object beam 7" is collected on CCD 5 with lens 4, computer 8 stores and analyzes an electrical signal from CCD 5.
When incident reference beam 6 enters with the angle of incidence A2 ' changed from its original angle of incidence A2 19, a reconstructed object beam 7" corresponding to this is generated.
In this way, the information stored on recording medium 3 can be reconstructed.
As reference beam 6 goes to the recording medium 3 where information is recorded, only the pattern of interference, created by object beam 7 and reference beam 6 whose angle of incidence is the same as that of reference beam used at the time of recording, satisfies Bragg's law to reproduce a single object beam. That is, object beam may be selectively reconstructed by controlling the angle of incidence A2 19 of reference beam 6.
In general, variation in angle of incidence A2 19 for another information recording is about 0.01.degree..
Reference beam 6 having an error not exceeding 0.001.degree. from desired angle A1 20, incident for original information recording, satisfies Bragg's law to generate a reconstructed object beam, etc.
The beam deflector 11 is constructed such that the incident reference beam 6 is incident at a predetermined angle A2, A2' maintaining planned accuracy (normally within 0.001.degree.).
However, when physical shaking occurs on recording medium 3 or the recording medium 3 is replaced after recording, recording medium 3' deviates from its original position and its axis 18' is changed from the original axis 18, as shown in FIG. 2.
In this case, an angle 19' that the incident reference beam 6 makes with the recording medium's axis 18' is also changed from original angle 19. In general, if its errors equals 19'-19&gt;0.001.degree. (whereas S denotes error, 19' denotes the angle that the incident reference beam makes with the recording medium's axis 18', and 19 denotes the original angle that the incident reference beam makes with original recording medium's axis 18), desired information isn't reconstructed.
In that case, it is unavoidable to control the beam deflector 11. When the minimum control step of beam deflector 11 is larger than the above-mentioned expression S, however, it is impossible to control the beam deflector 11.
Because the minimum control step of beam deflector 11 is the variation in angles of incidence A2 and A2', namely about 0.01.degree., the beam deflector 11 is in common use and has a disadvantage in case of frequent recording medium replacement.
In case the recording medium is replaced quite often, the beam deflector alone can't control the angle of incident reference beam with desired accuracy, thus not allowing the reconstruction of the desired information.